1. Field of the Invention
The present invention relates to a dark current component removing technology for observation image data.
2. Description of the Related Art
In recent years, electronic still cameras have been developed to mainly capture and record still images. An image capturing element used for a camera of this type has fixed pattern noise (dark noise) caused by variations in the crystal quality and the output characteristic of a material in each pixel. Therefore, if an output signal of an image capturing element is used unchanged, its dark noise is superimposed on an effective signal component, leading to degradation in the quality of a captured image.
To remove such dark noise, an electronic still camera having a noise reduction function exists. With this noise reduction function, an image capturing element is exposed with a shutter closed, and a resultant dark output is recorded in a memory when an image is captured.
Then, the image capturing element is exposed with the shutter opened, and the dark output stored in the memory is subtracted from a bright-time image signal obtained with this exposure.
With this method, dark noise that occurs with the same sensitivity and for the same exposure time is almost constant. Therefore, dark noise that occurs at the time of a dark output is canceled by a difference, whereby dark noise can be suppressed with high efficiency.
Dark noise increases with an electric charge storage time or sensitivity. Accordingly, the effect of further improving the quality of an image can be produced with this method at the time of long exposure and highly sensitive exposure. Accordingly, a noise reduction process is a function beneficial to a user of an electronic still camera.
However, since the noise reduction process requires a dark output obtainment time, continuous image capturing is hindered by a dark output obtainment process time in a method for obtaining a dark output immediately after image capturing. Additionally, a method for obtaining a dark output immediately before image capturing has disadvantages such as a shutter time lag caused by a dark output obtainment process, leading to the missing of a photo opportunity.
Accordingly, Japanese Published Unexamined Patent Application No. 2002-135661 discloses the technique for obtaining dark output data, which is longer by several stages than an exposure time determined at the time of metering, immediately before image capturing, and for using the data for a subsequent process. With this technique, a subsequent shutter time lag can be prevented from occurring, and continuous image capturing can be made by using dark output data after the data is obtained.
Additionally, Japanese Published Unexamined Patent Application No. 2005-159447 discloses the image capturing apparatus that controls a signal read from an image capturing element according to a relationship between the exposure time of the image capturing element and the time interval of image capturing, and can obtain a signal, the dark noise of which is suppressed, by executing a process for subtracting a dark-time signal, which is output from the image capturing element in a light-shielded state, from a bright-time signal that is output from the image capturing element in a light-unshielded state. With this apparatus, a high-quality image the dark noise of which is suppressed can be obtained while satisfying an image capturing interval at the time of interval shooting repeatedly made at predetermined time intervals.